Ultra high frequency amplifier



Oct. 22, 1963 R. H. BARTRAM 3,108,232

ULTRA HIGH FREQUENCY AMPLIFIER Filed July 6, 1960 2 SheetsSheet 1 INVENTOR RALPH H. BARTRAM BY FL. 2 rm A'I I'ORNEY Oct. 22, 1963 R. H. BARTRAM 3,103,232

' ULTRA HIGH FREQUENCY AMPLIFIER Filed July 6, 1960 2 Sheets-Sheet 2 INVENTOR RALPH H. BARTRAM BY FL. W

ATTORNEY United States Patent 3,193,232 ULTRA HlGlll FREQUENCY AMPLlFlER Ralph H. Bartram, Fresh Meadows, N311, assignor to General Telephone and Electronics Laboratories, line, a corporation of Delaware Filed July 6, 196%, der. No. 41,116 3 Claims. (fill. 33ll-44} This invention relates to amplifiers and in particular to amplifiers for use at ultra high frequencies.

In an article entitled A New Electron Tube: The strophotron by Hannes Alfven and Dag Romell, published in the Preceedings of the I.R.E., page 1239, vol. 42, No. 8, August 1954, there is described a rnultitransit electron tube oscillator. Improved versions of this tube are disclosed in my U.S. Patent 2,915,666 issued Decemher 1, 1959, and in my US. Patents 2,903,619 and 2,903,- 620, issued September 8, 1959. The present invention utilizes the strophotron principle to provide an eflicient voltage tunable UHF amplifier, and therefore a brief description of the strophotron oscillator will first be pres sented.

In one form of strophotron oscillator, a pair of elongated accelerator electrodes having cross sections defining branches of a hyperbola are positioned within an evacuated envelope. A pair of elongated reflector electrodes, also having cross sections defining branches of a hyperbola, are disposed between the accelerator electrodes and extend in the same direction as the accelerators. A cathode is mounted adjacent one end of the envelope in a slot in one of the reflectors while a collector electrode is located near the other end of the envelope. A load is connected across the reflectors. The accelerator and collector electrodes are maintained at a high positive potential with respect to the reflectors While the cathode is held at the same potential as the reflectors. A uniform magnetic field is established within the tube, the magnetic field vector pointing in a direction perpendicular to a plane passing through the longitudinal axes of the hyperbolic accelerator electrodes.

Under the influence of the magnetic field, electrons emitted by the cathode avoid interception by the accelerators and drift down the tube eventually striking the collector. When a radio frequency voltage is present across the reflectors, the electrons oscillate with changing amplitude in a plane parallel to the direction of the magnetic field. Those electrons which are emitted at such time that their instantaneous velocity vectors are substantially out of phase with the R.-F. electric field between the reflectors (termed unfavorably phased electrons) oscillate with increasing amplitude until they impinge upon one of the reflectors and are thus removed from the interaction region between the electrodes. Those electrodes which enter the interaction region at such time that their instantaneous velocity vectors are substantially in phase with the R.-F. electric field (termed favorably phased electrons) remain within the interaction region and induce currents in the reflectors. The favorably phased electrons oscillate with decreasing amplitude as they give up energy to the load thereby maintaining the R.-F. voltage across the reflectors.

The strophotron oscillator described above is highly efficient, has a relatively low output impedance, and is volt age tunable over a wide range of frequencies. Since these characteristics are highly desirable in an amplifier operating at ultra high frequencies, it is an object of this invention to provide a voltage tunable amplifying device utilizing the strophotron principle.

It is another object of the invention to provide an amplifier having an internal impedance permitting eflicient matching with a low impedance load.

Still another object is to provide an amplifier in which ice the product of the gain and tuning range exceeds the gain-bandwidth product of conventional amplifier tubes at ultra high frequencies.

A further object is to provide an amplifier for use at ultra high frequencies which is inexpensive to construct, small in physical size, and relatively light in Weight.

In the present invention there is provided a strophotron amplifier comprising an elongated accelerator having a uniform cross section defining a branch of a hyperbola. First and second separated reflector electrodes are so p0- .sitioned as to coincide with corresponding portions of the first and second asymptotes of the hyperbola branch. In one embodiment of the invention, the surfaces of the reflectors form planes as in the strophotron oscillator disclosed in my aforementioned Patent 2,915,666. in other embodiments the reflectors are hyperbolic and an additional hyperbolic accelerator electrode is provided in a manner similar to that shown in my Patents 2,903,619 and 2,903,620 disclosing improved strophotron oscillators.

In the amplifier of the present invention a cathode, maintained at a positive potential with respect to the reflectors, is mounted in a slot located at one end of one of the reflectors. A shield containing a screen grid is p0- sitioned adjacent the cathode in the interaction region between the reflectors. A control grid is interposed between the cathode and screen grid, means being provided for coupling an input signal across the grid and cathode electrodes. A collector electrode is mounted near the end of the electrode structure remote from the cathode and provisions are made for coupling a load across the reflectors. The collector, as well as the accelerator electrode, is maintained at a high positive potential with respect to the cathode. A magnetic field, having its vector pointing in a direction perpendicular to the plane defined by the longitudinal axis of the hyperbolic accelerator electrode and the intersection of the asymptotes of the hyperbola, is applied across the electrode structure.

Under the influence of the magnetic field, electrons emitted by the cathode avoid interception by the acce1erator and drift down the tube to eventually strike the collector. With no signal applied between the grid and cathode, these electrons oscillate with a constant amplitude sinusoidal component in the direction of the magnetic field vector. The electrons do not strike the reflectors since the reflectors are biased at a negative potential with respect to the cathode. Thus, the positive feedback loop present in the strophotron oscillator does not exist in the amplifier and the tube does not function as an oscillaamplifier and the tube does not function as an oscillator.

If it is assumed that a radio frequency voltage is impressed across the reflectors, the trajectory of a given electron will depend upon the phase of the R.-F. voltage at the instant the electron enters the interaction region. An electron which enters the incteraction region with its instantaneous velocity vector substantially in phase with the R.-F. electric field between the reflectors (that is, a favorably phased electron) will oscillate with decreasing amplitude as it drifts toward the collector, resulting in conversion of the energy of oscillation of the electrons to 1 .-F. energy. Conversely, an electron which enters the interaction region with its instantaneous velocity vector substantially out or" phase with the R.-F. electric field (that is, an unfavorably phased electron) Will oscillate with increasing amplitude converting R.-F. energy to elec tron oscillation energy. Since the current induced in the reflectors varies with the velocity of the electron and therefore its amplitude, there will be a net conversion of R.-F. energy to electron oscillation energy and the device will act as an attenuator.

When a signal is applied the control grid and cathode, the electron stream is current density modulated. if the accelerator voltage is adjusted to make the electron oscillation frequency equal to the signal frequency, the electron bunches in the modulated stream induce an R.-F. current in the load thereby producing an R.-F. voltage :across the load and reflectors. The resulting R.-F. field is in phase with the velocity vectors associated with the electron bunches and, therefore, the electrons in the bunches may be described as favorably phased. Those electrons that are emitted from the cathode between bunches are unfavorably phased and oscillate with increasing amplitude converting R.-F. energy to electron oscillation energy. However, since the number of unfavorably phased electrons is smaller than the number of favorably phased electrons, there is a net conversion of electron oscillation energy to R.-F. energy.

This mechanism depends upon the fact that the elec tron oscillation frequency and the modulation frequency are substantially equal. If the frequencies are sufiiciently different, favorably and unfavorably phased electrons will interchange their roles as they proceed down the length of the structure and there will be no gain.

The above objects of and the brief introduction to the present invention will be more fully understood and further objects and advantages will become apparent from a study of the following description in connection with the drawings, wherein:

FIG. 1 is a schematic perspective representation of one form of strophotron amplifier constructed in accordance with my invention in which the envelope has been omitted for clarity;

FIG. 2 is an end view of the electrode structure of the amplifier of FIG. 1;

FIGS. 3a and 3b are graphs showing the motion of electrons in the interaction region in the absence of an input signal; and

FIGS. 4a and 4b are graphs depicting the motion of favorably and unfavorably phased electrons in the interaction region respectively.

Referring to the schematic representation of the amplifier illustrated in FIG. 1 and the end view of the electrode structure of BIG. 2, there is shown first and second vertically displaced, electrically conductive, elongated reflectors 1t? and 11. Each reflector has a uniform cross section defining a branch of a hyperbola. The first reflector 10, having a longitudinal axis 12, forms the upper branch of a first hyperbola while the second reflector 11, having a longitudinal axis 13, forms the lower branch of the first hyperbole. Thus, the two reflectors are intersected by a first plane of symmetry y-z defined by the longitudinal axes 12 and 13.

Similarly, there is provided first and second horizontally displaced, electrically conductive, elongated accelerators 14- and 15 extending in the same direction as reflectors and 11. Each accelerator has a uniform cross section defining a branch of a hyperbola. The accelerators are symmetrically displaced about opposite sides of the first plane of symmetry so as to define left and right hand hyperbola branches; the left hand branch having a longitudinal axis 16 and the right hand branch having a longitudinal axis 17. Hence, the two accelerators are intersected by a second plane of symmetry x-z defined by longitudinal axes 16 and 17. Each reflector together with its adjacent accelerator defines a common hyperbola asymptote; thus there are four asymptotes 19, 20, 21, and 22.

The electrode structure is enclosed in an evacuated envelope 18 having a longitudinal axis defined by the intersection of the first and second planes. A uniform magnetic field B is established within the region bounded by the accelerators and reflectors, the magnetic field vector pointing in a direction perpendicular to the second plane of symmetry as indicated by the arrow. (Means for establishing this field are conventional and have been omitted to avoid complicating the drawing.)

The two accelerators l4 and are coupled through an isolating inductance 25 to an adjustable source of positive direct voltage 26. Reflector 11 is connected directly to a source of negative direct voltage 27 bypassed to ground by a capacitor 28 While reflector 10 is coupled to voltage source 27 through an inductance 7.9. Inductance 29 permits an R.-F. voltage to appear across reflectors it and ill while they are maintained at essentially the same D.-C. potential. As a result, the electric field established between the accelerators and reflectors creates a hyperbolic potential distribution within the electron interaction region.

A cathode 35 is mounted in a slot at one end of reflector 11 and is located to the right of longitudinal axis 13. A slotted, hyperbolically curved shield 36 supporting a screen grid 36a, maintained at a positive potential by a source of direct voltage 37 bypassed by a capacitor 38, is mounted in the interaction region adjacent cathode 35. A control grid 39, interposed between shield 36 and cathode 35, is connected to one input terminal 49 while the cathode is connected to the other input terminal 41. Cathode 35 is grounded and therefore is at a positive D.-C. potential with respect to the reflectors 10 and 11. An impedance matching network (not shown), which may be mounted within envelope 18, may be connected to terminals 49 and 41 to achieve a wide-band match of the signal source impedance to the input impedance of the amplifier.

A collector 43 is located in the interaction region at the end of the electrode structure remote from cathode 35. The surfaces of collector 43 are perpendicular to both the magnetic field vector and the electric field vector thereby increasing the electron collection efficiency. The advantages and characteristics of this particular orientation of collector 43 are described in greater detail in US. Patent 2,897,393 issued to Stanley A. Iorio, July 28, 1959. A pair of capacitors 45 and 46 couple reflectors l0 and 11 respectively to an externally connected load 47.

Electrons are emitted by cathode 35 and after passing through control grid 39 and screen grid 36a, drift under the influence of the magnetic field along a curved path toward the collector 43. With no input signal applied between the control grid 39 and cathode 35, the projection of this path on to the first plane of symmetry yz is a sinusoid of constant amplitude with a superimposed trochoidal motion. The electron path in the yz plane is shown at 50 in FIG. 3a with the trochoidal motion deleted for simplicity. Since reflectors 1t) and 11 are held at a negative potential with respect to cathode 35, the electrons do not strike the reflectors.

The projection of the electron path onto the second plane of symmetry x-z resembles a trochoid as shown at 51 in FIG. 3b. The sinusoidal frequency depicted in FIG. 3a is the oscillation frequency and is primarily determined by the electric field established between the accelerators l4 and 15. The trochoidal frequency (shown in FIG. 3b) is determined by both the magnetic and electric fields and is independent of the oscillation frequency. The magnetic field intensity is adjusted to a value at which electrons are prevented from impinging on the accelerators.

When a signal is applied to input terminals 40 and 41, the electron stream emitted by cathode 35 is current density modulated at the frequency of the amplified signal. If the accelerator voltage is adjusted to make the oscillation frequency equal to the modulation frequency, the tube amplifies the input signal and an R.-F. voltage appears across load 47. The electrons in the bunches produced by the modulating signal are favorably phased and decrease in amplitude as they proceed down the tube resulting in the conversion of electron oscillation energy to R energy. This is shown by the path 50a in FIG. 40. Those unfavorably phased electrons which enter the interaction region with their velocity vector substantially out of phase with the R.-F. field increase in amplitude as shown at 5% in FIG. 4a converting R.-F. energy to electron oscillation energy. Since there is a preponderance of favorably phased electrons due to the bunching action, there is a net conversion of electron oscillation energy to R.-F. energy. With a given amplitude of current density modulation, the R.-F. voltage across reflectors lii and 11 adjusts itself to the point where it is self-sustaining.

As described above, the R.-F. frequency is equal to the frequency of oscillation of the electrons in the y direction. If the frequencies are sufficiently different so there is a curnlative phase shift of 1r down the length of the structure, favorably and unfavorably phased electrons will interchange their roles during transit and there will be no gain. Actually, the product of the frequency difference and the transit time should be much less than 11' if high gain is to be achieved. Since the transit time is much longer than a period, this requirement makes the amplifier quite selective. However, the amplifier is voltage tunable over a wide range since the electron oscillation frequency can be varied considerably by merely changing the accelerator voltage.

The internal resistance of the strophotron amplifier is low enough to permit impedance matching to load 47 consistent with the desired tuning range. This may be compared with the conventional UHF triode in which the plate resistance is generally much larger than the load impedance.

The strophotron amplifier may also be viewed as a modification of the UHF triode. In both tubes, a modulated electron beam is injected from a cathode grid region characterized by a transconductance g and a grid resistance R In a triode used for broad band applications, where the plate load resistance R is small compared with the plate resistance R the power gain is In the strophotron amplifier, the mutual transconductance is effectively multiplied by twice the number of cycles n made by the electrons in the y-z plane as they drift down the electrode structure. This occurs because each bunch of electrons and each rarefaction in the interaction region induces a current in the reflectors corresponding to the R.-F. current modulation on the beam. Since half of the current is diverted to the internal resistance of the strophotron, which is low enough to permit impedance matching consistent with bandwidth, the strophotron gain becomes The cathode area of the strophotron amplifier is somewhat limited, resulting in a g for the strophotron which is somewhat less than the g of the triode. However, the product of the gain and tuning range of the strophotron amplifier exceeds the gain-bandwidth product of the triode because of the relatively large magnitude of the factor 21 It should also be noted that the oscillation frequency may be doubled by using the accelerators as one output terminal and the reflectors as the other rather than by simply coupling the output across the reflectors.

As many changes could be made in the above con struction and many different embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An amplifier comprising (a) an evacuated envelope including first and second vertically separated elongated reflectors having uniform cross sections in the form of a first hyperbola;

(b) first and second horizontally separated elongated accelerators having uniform cross sections in the form of a second hyperbola, said reflectors and ac- (c) electron emission means mounted adjacent one end of said first reflector, said electron emission means producing a continuous stream of electrons;

(d) means for maintaining said reflectors at a negative potential with respect to said electron emission means;

((2) collector means positioned adjacent the other end of said first reflector;

(1) means for maintaining said accelerators and collector at a positive potential with respect to said electron emission means; and

g) grid means mounted adjacent said electron emission means for controlling the stream of electrons emitted by said electron emission means, said stream of electrons being current density modulated in accordance with an input signal applied between said grid and emission means, the voltage between said electron emission means being selected to make the electron oscillation frequency substantially equal to the frequency of said applied input signal.

2. An emplifier comprising (a) an evacuated envelope including first and second vertically separated elongated reflectors having uniform cross sections in the form of a first hyperbola;

(b) first and second horizontally separated elongated accelenators having uniform cross sections in the form of a second hyperbola, said reflectors and accelerators being positioned to simulate the configuration of the asymptotes of said hyperbolas;

(c) a cathode mounted adjacent a slot in one end of said first reflector, said cathode producing a continuous stream of electrons;

(d) means for maintaining said reflectors at a negative potential with respect to said cathode;

(e) collector means positioned adjacent the other end of said first reflector;

(1) means for maintaining said accelerators and collector at a positive potential with respect to said cathode;

(g) a screen grid mounted adjacent said cathode in the region between said first and second reflectors; and

(h) a control grid positioned between said cathode and said screen grid for controlling the stream of electrons emitted by said cathode, said stream of electrons being current density modulated in accordance with an input signal applied between said control grid and cathode, the voltage between said accelerators and said cathode being selected to make the electron oscillation frequency substantially equal to the frequency of said applied input signal.

3. An amplifier comprising (a) an evacuated envelope situated within a magnetic field;

(b) first and second vertically separated elongated reflectors having uniform cross sections in the form of a first hyperbola, said first and second reflectors being oriented within said evacuated envelope so that a first plane passed through the longitudinal axis of said reflectors is parallel to the direction of said magnetic field;

(c) first and second horizontally separated elongated accelerators having uniform cross sections in the form of a second hyperbol-a, said first and second accelerators being oriented Within said evacuated envelope so that a second plane passed through the longitudinal axes of said accelerators is perpendicular to said first plane and equidistant from the longitudinal axis of said reflectors;

(d) a cathode mounted adjacent a slot in one end of said first reflector, said cathode being maintained at a positive potential with respect to said reflectors;

(e) a collector positioned adjacent the other end of said first reflector;

(f) voltage means for maintaining said accelerators and collector at a positive potential with respect to said cathode;

(g) a shield including a screen grid mounted adjacent said cathode in the region between said first and second reflectors;

(It) means for maintaining said shield at a positive potential with respect to said cathode;

(i) a control grid positioned between said cathode and said shield; and

(k) means for applying an input signal between said control grid and said cathode, the voltage between said accelerators and said cathode being selected to make the electron oscillation frequency substantially equal to the frequency of said applied input signal.

References Cited in the file of this patent UNITED STATES PATENTS Iorio July 28, 1959 Alfrn Feb. 9, 1960 Tube: The Strophotron, by Alfven and Romell, pages 1239-1241. 

1. AN AMPLIFIER COMPRISNG (A) AN EVACUATED ENVELOPE INCLUDING FIRST AND SECOND VERTICALLY SEPARATED ELONGATED REFLECTORS HAVING UNIFORM CROSS SECTIONS IN THE FORM OF A FIRST HYPERBOLA; (B) FIRST AND SECOND HORIZONTALLY SEPARATED ELONGATED ACCELERATORS HAVING UNIFORM CROSS SECTIONS IN THE FORM OF A SECOND HYPERBOLA, SAID REFLECTORS AND ACCELERATORS BEING POSITIONED TO SIMULATE THE CONFIGURATION OF THE ASYMPTOTES OF SAID HYPERBOLAS; (C) ELECTRON EMISSION MEANS MOUNTED ADJACENT ONE END OF SAID FIRST REFLECTOR, SAID ELECTRON EMISSION MEANS PRODUCING A CONTINUOUS STREAM OF ELECTRONS; (D) MEANS FOR MAINTAINING SAID REFLECTORS AT A NEGATIVE POTENTIAL WITH RESPECT TO SAID ELECTRON EMISSION MEANS; (E) COLLECTOR MEANS POSITIONED ADJACENT THE OTHER END OF SAID FIRST REFLECTOR; (F) MEANS FOR MAINTAINING SAID ACCELERATORS AND COL- 